Functional Morphology of Mechanoreceptors in Astigmatic Mites

Although the mechanoreceptor organs of astigmatic mites are used extensively as taxonomic characters, knowledge of their fine structure is minimal. Mechanoreceptors include setiform sensilla and cupules, and light microscopy, scanning electron microscopy and transmission electron microscopy were used to investigate their morphology. Setiform sensilla are movable and arise from a socket of thin procuticle. The setal base is anchored in the socket by radiating suspension fibers that connect the socket to a thin layer of more heavily sclerotized cuticle surrounding the setal base. Two dendrites terminating in tubular bodies are associated with the sensillum. Four pairs of cupules are found on the lateral margin of the opisthosoma. The cupule consists of a thin layer of cuticle (covering membrane) that overlies a “cup‐like” depression within the remaining cuticle. A layer of densely‐packed, thick fibers radiate dorsally through the “cup”, attaching to its sides and centrally associated with a vacuole‐like structure. A dendrite containing a single tubular body enters the cup through an opening in the basal cuticle and connects with the vacuole‐like structure rather than with the covering membrane. It is postulated that distortions of the covering membrane are relayed through the fibers and thereby compress the vacuole‐ like structure which in turn deforms the tubular body and stimulates the dendrite. The cupules are thought to be cuticular receptors that register strains in the cuticle caused by gravity, vibrations, movements of the mite, and/or internal changes in pressure.